Intermittant P0411 on my 2000 V6 Mustang


Jun 17, 2020
Rochester NY
Hello, all, I have been battling an intermittent P0411 "Service Engine"/CEL light issue for some time.
Done the usual; replaced the smog pump. Reduced the frequency of CEL, but not completely.

So here's the story. Have a quick read, and comment.

History: this car was purchased in CT about 5 years ago, and has spent most time since in Upstate New York, where it snows a lot and they use lots of salt on the roads.
Would get P0411 with the CEL pretty regularly. So when I replaced the smog pump a couple years ago, I found that the wiring harness from the relay to the pump had corroded wiring. Obviously, the previous owner had been dealing with the issue. It looked like someone had put a tap on the wires to monitor whether the smog pump was being turned on, and the break in the insulation had let salt water in to rot the wires. So I fixed the wiring harness, put in the new smog pump, and things seemed hunky-dory...

...Until the CEL came on a few days later, code P0411. Not regularly; every so often. Tough to diagnose something that show up only every so often, and usually when you're not thinking about it.

So, I thought, why intermittent? Well, the P0411 DTC gets checked in the first minute or so after start, based whether fresh air from the smog pump into the exhaust stream drives the downstream O2 sensor to "lean". Well, if that signal doesn't go "lean" (low voltage), the the code gets set. So to pass the "emissions ok" test that the car does, there needs to be extra air (functioning smog pump and air delivery pipes), a "lean" signal from the O2 sensor (a good sensor), and a good signal path to the engine control computer.

Now, the smog pump can have an intermittent motor. The one I replaced ran ok when I tested it offline, but I left the new one in.

One thing that can cause the signal to fail to show "lean" is a ground fault, or some resistance in the ground path between the O2 sensor and the rest of the vehicle - like a grotty ground connection. Something which keeps the signal from being driven sufficiently low, every so often.

So I went around and looked at EVERY screw that connected a wire to the chassis. Every single one had corrosion on it. So I used lots of wire brush on a dremel plus contact cleaner to clean up the connection points, and replaced all the screws with clean new hardware. The crimp points of the wiring to the grounding connector itself are suspect, too.

So far, the CEL has NOT come back on, after several months. We will see once the car gets more exercise now that the snow is gone.

Now, the vehicle is 22 years old, and I suspect that the O2 sensors and cat converters are still original (180k miles on the odometer). The O2 sensors might be marginal. I plan to check all the O2 voltages, plus run some CataClean through the system this spring. Also renewing the grounding connectors, to make sure there's good connection to the wiring harnesses (plural) to the grounding connector. Shiny is good.

It's possible, particularly in older vehicles driven in salt-using states (like the NorthEast), that part of the P0411 problem might decay in the ground connections to the chassis. It's NOT on the diagnostics list, and is a relatively easy (and cheap) item to check out if someone is experiencing every-so-often P0411 DTC issues.

Comments appreciated.
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Update: the demon code is back this spring. Next steps is a can of Cataclean through the fuel system and some (ouch!) premium fuel, on the theory that the catalytic converter efficiency is marginal, but nor bad enough to trigger the P0420 code, and that the Cataclean perks up the O2 sensors.
Stay tuned...
Heres what I would do seeings your issue seems to be power or ground related if your exhaust isnt excessively rusty or has exhaust leaks as an excessively rusty exhaust will inhibit a good ground path for the 02 sensors and leaks add oxygen into the stream leaning out the AF reading causing the engine to run excessively rich to compensate..

Sometimes the issue can be caused by a clogged fuel filter too.....

If the exhaust is good and leak-free pull the battery and the computer and have the battery load tested then open up the computer and check to see if there are any internal damages that could be seen by eye.....

I send all my ECU's to ECU Exchange...They're one of the best and the fastest in the business and know their stuff very well as when they diagnose your issues and find a problem that could lead to the ECU being re-damaged if reinstalled they tell you where to look for issues in the wiring...

Take the battery positive and negative out and replace them .

Clean up all of the grounds especially the HEGO ground coming off of pin #49 on the ECU plug thats usually attached to either the firewall or the intake manifold...

There are grounds around the battery area attached to either the core support or the unibody that are used for the computer on pins #40 and pin #60 make sure they're clean and grounded good..

The power to power the HEGO's are fed through the ignition switch attached to the steering column too and may need replacing..

Good Luck
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Update: the demon code is back this spring. Next steps is a can of Cataclean through the fuel system and some (ouch!) premium fuel, on the theory that the catalytic converter efficiency is marginal, but nor bad enough to trigger the P0420 code, and that the Cataclean perks up the O2 sensors.
Stay tuned...

Heres what I do every time I change the oil and only takes maybe 20-30 minutes to do...

Before I drop the oil I pull my 02 sensors and I put them in a glass of lemon juice to fully clean them while I do the oilchange then when done with the oilchange I rinse the 02 sensors in a vinegar bath then rinse off the vinegar with rubbing alcohol and reinstall after blowing them dry with a heatgun...

The lemon juice works quickly at attacking also loosening up and removing the carbon build-up and the vinegar makes the electrode tips on sparkplugs really clean as the lemon juice and separate vinegar bath / alchohol rinse trick works great on sparkplugs too and sometimes needs a lil light help from a wire brush but never use a wirebrush on an 02 sensor...LOL


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(Bump) an update. Took the vehicle to a new mechanic (ex-Ford, Master Mechanic rated, running his own shop now)
for another totally unrelated issue, and asked that he have a crack at the P0411 issue.
Bingo, in an hour of real diagnostics, he found that the EGR Vacuum Solenoid Valve (Ford p/n F57E9J459-CA) was defective.
The guy took the time to show me that it was leaking, not holding the vacuum that it should; and THAT was the cause of the intermittent P0411.
Had the car back for only a day; the code is gone, and hasn't come back, but it will take a few more days of driving for me to be convinced.
Will post again after I put a couple hundred miles and a dozen or so start/stop cycles, but I am for the first time hopeful.
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(bump) Update #2. Close, but no cigar. Two days later, the code is back. (plus another issue, but not important). This time it's the Vacuum Control Solenoid (Ford part #6L3Z-9H465B) which lives right next to the (now, new) EGR Vacuum Solenoid Valve (see my earlier bump post). Mechanic sheepishly told me he stopped the diagnostic process after finding the failed EGR Vacuum Control Solenoid, and that the OTHER Vacuum Control Solenoid was completely inoperative. Ford is NOT terribly consistent (or descriptive) in naming these solenoid valves. Makes it tough to interpret the vacuum diagram for the smog pump (oops, Secondary Air Injection).
Yes, BOTH these parts need to be functional in order to get the air from the smog pump into the exhaust to drive the O2 sensor voltage low on command. Both of these parts are probably original on my Pony, so that makes them 20+ years old, so I'm not totally surprised.
Now, I have no skills in debugging vacuum lines, and have no diagnostic tools (like vacuum pump & gauge) for this topic, so I'm leaving this for the mechanic to puzzle through. This time he's going through the ENTIRE vacuum diagnostic process and waiting for all the drive monitors to show green (plus finish the other little problem) before he turns the car back over to me. I expect the car back in a day or so; I'll report back on the outcome.
So here are a couple of potential culprits to consider if you're chasing the P0411 fault code.
Stay tuned.....
<edit>(try to append diagram on Secondary Air Control vacuum diagram)
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